The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is para-xylene, the principal feedstock for polyester, which continues to enjoy a high growth rate from large base demand. Ortho-xylene is used to produce phthalic anhydride, which supplies high-volume but relatively mature markets. Meta-xylene is used in lesser, but growing, volumes for products such as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but is usually considered a less-desirable component of C8 aromatics.
Among the aromatic hydrocarbons, the overall importance of xylenes rivals that of benzene as a feedstock for industrial chemicals. Xylenes and benzene are produced from petroleum by reforming naphtha, but not in sufficient volume to meet demand; thus, conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Often toluene is de-alkylated to produce benzene or selectively disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered.
Aromatics complexes producing xylenes are substantial consumers of energy, notably in distillation operations to prepare feedstocks and separate products from conversion processes. The separation of xylenes from a feedstock in particular offers substantial potential for energy savings. Energy conservation in such processes would not only reduce processing costs, but also address current concerns about carbon emissions.
In addition to producing xylenes, valuable fuel gas is generated during the catalytic conversion of xylenes in an aromatics complex. A portion of this fuel gas is recoverable in a xylene isomerization unit.
The xylene isomerization units typically include a deheptanizer and a stabilizer. The current designs for xylene isomerization units utilize at least two recycle loops between the deheptanizer and the stabilizer.
In the first recycle loop, at least a portion of the overhead vapor from the stabilizer is recycled back to the deheptanizer. This will result in this portion of the vapor being re-condensed, re-flashed, and ultimately re-compressed in the same separation process since it is passed back to the deheptanizer and will pass through the same separation equipment.
The second recycle loop is formed between a receiver and a vent drum where the chilled liquid from the vent drum enters the hotter, low pressure receiver and re-flashes into vapor to the compressor. This requires the same compounds to be re-compressed and cooled once again.
It is believed that the current design is inefficient at least because both of the recycle loops lead to undesirable re-processing of the same material at the expense of equipment capacity and utility cost. Energy conservation in such processes would not only reduce processing costs but also would address current concerns about carbon emissions.
Therefore, there is a need to provide a xylene isomerization process which may be carried out more efficiently.